1. Field of the Invention
This invention relates to a method for modifying poly(glycidyl nitrate) that allows the poly(glycidyl nitrate) to be stably cured through the use of aliphatic polyisocyanates. This invention further relates to the production of solid energetic compositions, such as propellants, explosives, pyrotechnics, and gas generants, comprising poly(glycidyl nitrate) binders.
2. State of the Art
Solid high energy compositions, such as propellants, explosives, pyrotechnics, and gasifiers, and the like, generally comprise solid particulates, such as fuel particles, oxidizer particles, or a combination of both, dispersed and immobilized in a cured binder matrix.
In recent years, energetic polymers have been developed and evaluated as potential replacements for inert polymeric binders in cast propellant systems, explosive compositions, and pyrotechnics. The substitution of an energetic polymer for an inert polymer in a conventional pressable or extrudable explosive composition generally increased the detonation pressure and detonation velocity of the explosive.
Poly(glycidyl nitrate) (also known as “PGN” and “polyGLYN”) has been known and recognized for years as a possible energetic polymer suitable for use in propellants, explosives, pyrotechnics, gas generants, and the like. PGN binders are commonly synthesized by preparing a difunctional glycidyl nitrate polymer and curing the PGN with a polyfunctional isocyanate having a functionality of greater than about 2.3 to give urethane cross-linked polymers. Aromatic and aliphatic polyisocyanates have been selected as the curing agents.
Although glycidyl nitrate prepolymers have a satisfactory shelf life, it is known that aliphatic polyisocyanate cured PGN inherently de-cures when stored at room temperature for prolonged periods. If precautions are not taken, over time, current PGN can de-cure to the point of reverting to a pourable liquid. Accordingly, special care must be taken in the handling and storing of energetic compositions containing PGN cross-linked using aliphatic polyisocyanates. The special care required to avoid a de-curing problem has impeded the widespread use of PGN as a binders despite its attractive energetic properties.
One solution to this de-curing problem is the replacement of the terminal nitrate ester groups of PGN with hydroxyl groups. This solution was first put forth by N. C. Paul et al. An Improved polyGLYN Binder Through End Group Modification, ICI Explosives (1998). The article indicates the de-curing problem as being caused by the proximity of the terminal hydroxyl groups of the polymer to nitrate ester groups. The authors conclude that the de-curing problem is an inevitable consequence of the end group structure. To overcome this problem, the article describes a two-step process (illustrated below) that modifies the end groups by removing the adjacent nitrate esters and replacing the nitrate ester groups with hydroxyl groups by base catalyzed hydrolysis. In particular, the first step of the ICI process involves an epoxidation of the terminal hydroxyl group and the adjacent nitrate ester in the presence of KOH and EtOH, with dichloromethane acting as the solvent. The material is then isolated between the steps with removal of the solvent. The material is then redissolved in tetrahydrofuran (THF) in the presence of sulfuric acid and heated so as to open the epoxide ring thus providing a terminal hydroxide group in place of the original nitrate ester. Aging tests have shown that this technique is successful in preventing de-cure of the polymer.

However, the ICI process as described by Paul et al. has drawbacks in that it has two discreet process steps, causing additional expense and chemical waste in production. Accordingly, it would be an improvement in the art to generate the final product of the above-described ICI reaction using fewer chemical process steps and fewer solvents.